The internet could transform the practice of epidemiology, with its ability to communicate with large numbers of people over broad geographic areas, its dramatically lower costs compared with older forms of communication, and its convenience for maintaining contact with study participants. Despite this promise, thus far little research has been conducted to evaluate the efficacy and validity of conducting prospective cohort studies using the internet for recruitment and follow-up. We propose to conduct a pilot study to evaluate risk factors for delayed fecundability using the internet. This exploratory study will be conducted in Denmark, which has one of the highest prevalences of internet usage in the world, and maintains extensive registry data on a variety of health exposures and outcomes that are linkable using a national identity number assigned to every Danish citizen at birth. The study will assess the feasibility and validity of using the internet for recruitment and data collection. Secondarily, it will evaluate several potential risk factors, including caffeine, smoking and medication use, in relation to delayed time to conception. In addition, by linking with registries that contain information on pregnancy outcomes, we will evaluate several pre- conception and early pregnancy exposures in relation to risk of miscarriage and other birth outcomes. The proposed study is unique because most studies of fecundability have evaluated the time to conception retrospectively among women who become pregnant, a method that may result in selection and recall bias. The internet will allow us to identify women who are contemplating a pregnancy, a challenging population to target using traditional epidemiologic methods. Using targeted internet advertisements, we will recruit a cohort of 5,000 women age 18-40 who have discontinued birth control within the past month, or who plan to attempt a pregnancy within the next six months. After a short run-in period to verify a participant's identity and willingness to participate in a year-long study, participants will be followed using web-based questionnaires every two months, until conception occurs, or for as long as one year. The proposed internet- based methodology is novel but largely untested in observational epidemiologic settings. Such a design could have far-ranging implications for the practice of epidemiology in general. If successful, we envision this study to be the first of more elaborate studies that employ the internet to enroll cohort members into long-term follow-up studies. [unreadable] [unreadable] [unreadable]